In integrated photonic devices, such as integrated spectrometers, a photonic circuit may be provided on an image detector. For example, a plurality of grating couplers may couple light from a waveguide towards an image detector for detection.
For example, U.S. Pat. No. 6,226,083 discloses an integrated-optic spectrometer for analyzing the composition of light reflected off a sample under analysis. The integrated-optic spectrometer comprises a waveguide, fabricated onto an oxidized substrate, and an array of diffraction gratings and a detector array. The detector array is mounted on the waveguide to receive the light of different wavelengths diffracted by the grating array, e.g. such as to enable the analysis of different discrete wavelengths. The diffraction gratings each comprise a series of grating lines and are constructed to provide transmission of wavelengths not diffracted by the diffraction grating. The input light is guided through the waveguide and discrete wavelengths are diffracted by the diffraction gratings onto the photodiode detector array which in turn measures the intensity of the light at the discrete wavelengths for determining composition, while transmitting non-diffracted wavelengths through the waveguide.
In such a system, the diffraction gratings may have the shape of a square or rectangle, with grating lines at right angles to the direction of propagation of light within the waveguide, resulting in the light being coupled out of the waveguide in a direction that is about perpendicular to the plane of the waveguide, e.g. toward the photodiode detector array.
However, it is a disadvantage of such systems that each grating coupler may typically have a low out-coupling efficiency. Furthermore, this may adversely affect the signal to noise ratio for the signal detected by the individual pixels. It is known that, to increase the coupling efficiency of a grating coupler, its length may need to be increased. However, this implies that also the photodiode detector array should be chosen such that the individual detector pixel elements of the array are sufficiently large, e.g. such that the length of the gratings about matches the pixel pitch. This may put large constraints on the choice of imager from commercially available imaging solutions, or might even require the development of expensive custom arrays.